The present invention generally relates to semiconductor processing, and in particular to a system for monitoring and controlling the etching of openings in a phase shift mask.
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there have been, and continue to be, efforts toward scaling down device dimensions (e.g., at sub-micron levels) on semiconductor wafers. In order to accomplish such high device packing densities, smaller features sizes and more precise feature shapes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry, such as corners and edges, of various features. When feature sizes become so small that they approach the wavelength of the exposure light used in semiconductor manufacturing, complex exposure techniques including complimentary phase shift masking may be employed. In complimentary phase shift masking, light passing through one or more masks may be phase shifted to facilitate selective interference and cancellation of light waves. The ability to control the phase shift of the light passing through a mask is important to achieving the desired critical dimensions on the chip.
The masks employed in semiconductor fabrication that utilize complimentary phase shift masking may include a quartz layer coated with a chrome layer. The quartz layer allows light waves to pass through, while the chrome layer prevents light waves from passing through the mask. Thus, either a positive or negative of the pattern to be projected onto a chip being fabricated is processed into the chrome layer on a complimentary phase shift mask. The depth and/or width of the openings (apertures) in the complimentary phase shift mask enable light passing through the apertures to be phase shifted.
The process of manufacturing masks may consist of hundreds of steps. One such step is depositing a chrome layer on a clean quartz layer (substrate). Once deposited, openings (apertures) are etched into the chrome layer. Controlling the width and depth of the openings etched into the chrome layer and controlling the width and depth of trenches carved into the substrate is required to enable controlled phase shifting of light that will pass through the mask. Conventional mask fabrication methods may not provide fine enough control of the aperture etching process and thus desired phase shifting may not be achieved. Thus, a system and method for controlling the aperture etching process is required.
The process of manufacturing semiconductors, (integrated circuits, ICs, chips), employing complimentary phase shift masks typically consists of more than a hundred steps, during which hundreds of copies of an integrated circuit may be formed on a single wafer. Generally, the process involves creating several patterned layers on and into the substrate that ultimately forms the complete integrated circuit. The patterned layers are created, in part, by the light that passes through complimentary phase shift masks. Thus, processing the positive or negative of the pattern into the mask is important in fabricating the chips.
The requirement of small features with close spacing between adjacent features requires sophisticated manufacturing techniques, including high-resolution photolithographic processes such as complimentary phase shift masking. Fabricating a semiconductor using such sophisticated techniques may involve a series of steps including exposing the photo resist one or more times to one or more light sources (where the phase of the light may be shifted). In conventional lithography, an exposure is performed using a single mask where the photo resist is exposed by a single radiation source. The resolution, which is typically defined as the smallest distance two features can be spaced apart while removing all photo resist between the features, is equal to:
D=k1*(lambda/NA)
where d is the resolution, lambda is the wavelength of the exposing radiation, NA is the numerical aperture of the lens, and k1 is a process dependent constant typically having a value of approximately 0.7. While resolution may be improved by decreasing the wavelength or by using a lens with a larger NA, decreasing the wavelength and increasing the numerical aperture decreases the depth of focus (since depth of focus is proportional to lambda/NA), which creates additional problems. Thus, several techniques have been developed to enhance the resolution of conventional lithography to enable formation of patterned resist layers with smaller dimensions than those achievable with conventional methods. For example, phase-shifted masks (PSM) have been developed. In a PSM mask, features are surrounded by light transmitting regions that shift the phase of the transmitted light compared to the feature. Masks may be constructed to shift the phase of the light varying amounts, including, but not limited to, 30 degrees, 60 degrees, 90 degrees, and 180 degrees. In this way, the diffraction fringes at the edges of the features can be effectively cancelled, resulting in a better image contrast.
The resolution of both conventional and enhanced resolution lithographic processes is better for periodic features, such as those found in memory devices (e.g. DRAMs) because a greater percentage of the exposing radiation is contained in the diffraction nodes of the periodic structures compared to that contained in the diffraction nodes of isolated features. For example, prior art FIG. 15 illustrates an aerial plot of intensity under a mask 800 having an isolated feature 802 and periodic features 810, 812, and 814 having a dimension near the resolution limit of the process. The contrast (difference in intensity) between masked and unmasked regions is much greater for the periodic features 810, 812 and 814 (curve 806) than for the isolated feature 802 (curve 808). Thus, for a given combination of exposing conditions, at some dimension, isolated feature 802 cannot be resolved simultaneously with the periodic features 810, 812 and 814 that are within the resolution limit of the process.
To alleviate the problems associated with isolated features in complimentary phase shift masking, complementary features are added around the isolated device features on a first mask to produce a periodic structure that allows for improved resolution of the lithographic process. The effects created by the complementary features may require the light passing through the features to have its phase shifted. Such a shift may be accomplished by varying the width and/or the depth of the opening through which the light passes.
In a positive photo resist method, the complementary features are then obliterated by exposure to light passing through a second mask prior to forming the patterned resist layer. The second mask also provides for improved contrast that enables more precise feature shapes. To take advantage of complimentary phase shift masking, and removal of unwanted complimentary structures, precise control of the depth and/or width of the openings in the complimentary phase shift masks is required. If the depth and/or width of the opening is not precisely controlled, then the phase shifting, diffraction and cancellation processes employed in complimentary phase shift masking will not lead to a desired cancellation of light and the isolated features will not benefit from the improved contrast and resulting improved quality.
The complimentary phase shift masking discussed above is possible because light passing through one or more apertures (apertures) on a mask employed in chip manufacturing is diffracted. Diffraction is a property of wave motion, in which waves spread and bend when passed through small apertures or around barriers. A mask may have many such apertures and barriers. The bending and/or spreading of the light waves is more pronounced when the size of the aperture or the barrier approximates or is smaller than the wavelength of the incoming wave. With feature sizes approaching and becoming smaller than the wavelength of the exposing light, the apertures and/or barriers on the mask have thus become closer to the wavelength of the exposing light. Thus diffraction in chip manufacturing has become more pronounced, which can lead, for example, to rounded features and features that do not have a desired size and/or shape. For example, in prior art FIG. 16, a light source is directing light waves 1620 at a mask 1622. Some of the light waves 1620 pass through an aperture 1626 that is close to the size of the wavelength of the light waves 1620. The mask 1622 has been designed to develop a region 1638 on a photo resist layer 1624, so that two desired features 1642 and 1644 can be formed. The features 1642 and 1644 are desired to be rectangular, with substantially square edges. The aperture 1626 is small because the desired features 1642 and 1644 are correspondingly small.
With conventional lithography, the light waves 1620 may pass directly through the aperture 1626, exposing the region 1638, but the light waves 1620 may also be diffracted as illustrated by light waves 1628, 1630 and 1632. The diffracted wave 1628 has exposed a region 1634 and the diffracted wave 1630 has exposed a region 1636. Neither region 1634 nor region 1636 were intended to be exposed. Further, diffracted wave 1632 has exposed a triangular area 1640. Thus the desired feature 1644 may not have a substantially square edge due to the undesired region 1640 being exposed by the diffracted wave 1632. Complimentary phase shift masking mitigates the diffraction problems described above by accounting for and counter-acting the diffraction effects noted above.
A theory explaining diffraction is that each point of a wave on a flat wave front may be a source of secondary, spherical wavelets. Before reaching a barrier or aperture, the secondary wavelets may add to the original wave front. When the wave front approaches an aperture or barrier, the wavelets approaching the unobstructed region pass through the barrier, while other wavelets do not pass. When the size of the aperture approaches the wavelength of or is smaller than the wavelength of the incoming wave, only a few wavelets may pass through the aperture. The wavelets that pass through the aperture or around the barrier may then be a source of more wavelets that expand in all directions from the point of the obstruction, and the shape of the new wave front is curved. The wavelets of these diffracted, or bent, waves can now travel different paths and subsequently interfere with each other, producing interference patterns. The shape of these patterns depends on the wavelength and the size of the aperture or barrier. Diffraction can be thought of as the interference of a large number of coherent wave sources, and thus, diffraction and interference are substantially similar phenomenon.
To achieve desired interference, which leads to cancellation of undesired light waves, complimentary phase shift masks, which are well known in the art, are employed in manufacturing chips. Similarly, to enable smaller isolated feature sizes, complimentary phase shift masks are employed in manufacturing chips. Precise control of the depth and/or width of the openings in a mask employed in complimentary phase shift masking is required to enable the control of the phase shifting and resulting cancellation and interference that enables the smaller feature sizes with improved feature shapes. Thus, an efficient system and/or method to monitor and control the fabrication of the openings in complimentary phase shift masks is desired to increase fidelity in image transfer.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description presented later.
The present invention provides a system that facilitates monitoring and controlling the fabrication of openings (apertures) in complimentary phase shift masks employed in semiconductor manufacturing. Controlling the mask fabrication process with runtime feedback provides superior mask fabrication as compared to conventional systems and thus facilitates achieving smaller feature sizes with improved shapes via more precise control of phase shifting of light passing through the complimentary phase shift mask. An exemplary system may employ one or more light sources arranged to project light onto one or more apertures on a mask being fabricated and one or more light sensing devices (e.g., photo detector, photodiode) for detecting light reflected by the one or more apertures. The light reflected from the one or more apertures is indicative of at least one parameter of the mask fabrication process (e.g., depth of opening, width of opening). The depth and/or width of the apertures are important to the fidelity of the image transfer process due to effects on phase shifting and diffraction, and thus monitoring the depth and/or width of the apertures in the masks enables fabricating higher quality complimentary phase shift masks as compared to conventional systems.
A diffraction grating is an optical device that is used to determine the different wavelengths or colors contained in a beam of light. The apertures in a complimentary phase shift mask may operate, at least in part, similarly to a diffraction grating in that light will be reflected and dispersed when directed onto an aperture. A diffraction grating may include a reflecting surface, on which numerous narrow parallel grooves have been etched close together. A mask may contain numerous apertures etched closely together, which similarly will reflect and diffract light. A beam of light directed at such a surface is scattered, or diffracted, in all directions at each such aperture. Such scattering will be affected by the depth and the width of the apertures etched in the mask. The light waves reinforce each other in certain directions and cancel out in other directions, creating unique signatures for different wavelengths and/or angles of incidence of the light directed onto the mask.
One or more etching components may be employed in fabricating a particular mask. It is to be appreciated by one skilled in the art that any suitable etching components may be employed with the present invention. The etching components are selectively driven by the system to etch the openings in the mask to a desired depth, shape and/or width. The etching process is monitored by the system by comparing signatures generated by the light reflected by the mask to desired signatures. By comparing desired signatures to measured signatures, runtime feedback may be employed to more precisely control the aperture etching and as a result more optimal aperture etching is achieved, which in turn increases fidelity of image transfer, because more precise phase shifting and the resulting interference and cancellation may be enabled.
In accordance with an aspect of the present invention, a system for monitoring and controlling aperture etching in a complimentary phase shift mask is provided. The system includes etching components operative to etch apertures in the mask and an etching component driving system for driving the one or more etching components. The system also includes components for directing light on to the apertures being etched in the mask and a measuring system for measuring aperture parameters based on light reflected from the apertures. The measuring system includes a scatterometry system for processing the light reflected from the one or more apertures and a processor operatively coupled to the measuring system and the etching component driving system. The processor receives aperture data from the measuring system and uses the data to at least partially control the etching components to regulate the etching of the one or more apertures.
Another aspect of the present invention provides a method for monitoring and controlling aperture etching in a complimentary phase shift mask. The method includes etching apertures on the mask and while such apertures are being etched, directing light onto at least one of the apertures and collecting light reflected from the apertures. The reflected light is analyzed to determine parameters like the depth and/or width of the aperture via scatterometry means. In response to the analysis of the reflected light, the etching performed by the etching component is controlled to improve the etching of the apertures in the mask.
Still another aspect of the present invention provides a method for monitoring and controlling aperture etching in a complimentary phase shift mask. The method includes using etching components to etch apertures in the mask, determining the acceptability of the apertures etched in the mask and using coordinating control of the etching components to more optimally etch the apertures in the mask.
Yet another aspect of the present invention provides a system for monitoring and controlling a process for etching openings in a complimentary phase shift mask. The system includes means for sensing the depth and/or width of apertures on the mask, means for etching apertures on the mask and means for selectively controlling the means for etching.
To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.